The vast array of splice variants of mu opioid receptors (MOR-1) gene (Oprm1) can be divided into two groups based upon the promoters responsible for their generation. The primary promoter is associated with exon 1 and generates a large number of traditional 7 transmembrane domain receptors. The second promoter, associated with exon 11, located approximately 30 kb upstream of exon 1, generates a number of truncated, 6 transmembrane domains. Using a novel radioligand 125I-BNtxA synthesized in our laboratories, we recently reported a novel exon 11-associated binding site in a triple KO mouse lacking all exon 1- containing MOR-1 splice variants as well as delta and kappa1 receptors, that was lost in the exon 11 KO mice. IBNtxA is an effective analgesic, with a potency 10-fold greater than morphine. This analgesia persists in the triple KO mice, but is lost in the exon 11 KO mice. Despite it potent analgesic actions, IBNtxA lacks respiratory depression, significant constipation, physical dependence or reward. It shows no cross tolerance to morphine and can be given to morphine-dependent mice without a decrease in its own analgesic actions or the precipitation of withdrawal. Thus, this ligand avoids many of the problematic side-effects seen with traditional opioids by targeting truncated 6 transmembrane domain splice variants of the mu opioid receptor MOR-1(6TM/E11). I propose to use it as a lead compound to design a library of opioid analgesics. In spite of its favorable pharmacology, its selectivity for the new target over the traditional ones is only modest and can be improved. The goal of this project is to obtain selective and potent 6TM/E11 analgesics, establish an SAR and generate useful biochemical probes to study the biochemistry/molecular pharmacology of these sites. Analogs will include compounds based upon the 4,5-epoxymorphinan scaffold of IBNtxA and the morphinan scaffold (4,5-epoxymorphinans lacking the ethereal oxygen bridging rings A and C). Preliminary data suggests that an aryl amido at the 6-position of the opiate coupled with an iodine at the 3 or 4 position of the aryl enhances the affinity for the 6TM/E11 site. We will explore the chemical space around the 6 position of the 4,5-epoxymorphinan scaffold with various substituents. Other compounds include substituents on the tertiary nitrogen atom, the14-OH, and using aryl amido- epoxymorphinans with a double bond between 7,8 position. Finally, aryl amido-morphinans will also be synthesized. All synthesized compounds will be characterized for selectivity using in vitro radioligand binding assays and useful compounds will be evaluated in vivo.